The present invention relates to a multi-layer headbox for a paper or board machine including a dilution flow, i.e., a dilution headbox, a method for combining a flow of dilution liquid with flows of stock from inlet headers of a multi-layer headbox of a paper or board machine at a location between the inlet headers and a slice cone of the headbox and a method for regulating the basis weight of a web upon its discharge from a multi-layer headbox of a paper or board machine.
With respect to the prior art, reference is made to the constructions described in European Patent No. 0 634 523 and in German Patent Publication Nos. DE 44 35 860 and DE 44 16 909. These publications generally describe the controlled introduction of a dilution liquid into a stock flow in connection with a multi-layer headbox in order to regulate the basis weight of a multi-layer web formed from the headbox across the web width.
When multi-layer paper is produced by a multi-layer headbox, one of the main requirements of the paper is purity of the layers forming the same. This have proven to be difficult to achieve in the prior art multi-layer headboxes.
It is an object of the present invention to provide a new and improved multi-layer headbox which provides suitable purity of the layers of the multi-layer paper produced thereby.
It is another object of the present invention to provide a novel construction of a multi-layer headbox which is suitable both for paper machines and board machines.
It is another object of the present invention to provide a novel method for combining a dilution liquid with flows of stock from inlet headers of a multi-layer headbox.
It is another object of the present invention to provide a novel method for regulating the basis weight profile of a web across the web width.
In order to achieve these objects and others, the multi-layer headbox in accordance with the invention comprises separate inlet headers for each different stock concept and a common distribution plate arranged after the inlet headers in the flow direction of the stock. A dilution liquid is distributed through the distribution plate to one or more desired locations along the web width and, preferably at the same time, into all of the layers of the web at each of those locations in order to regulate the basis weight of the web across the web width. Further, preferably the same amount of dilution liquid, which is favorably dilution water, is passed into each layer.
In the construction of a multi-layer headbox in accordance with the invention, only one valve is needed for regulation of the dilution liquid in each zone of regulation across the width. As such, the construction is less expensive and simpler, compared with prior art constructions. In the construction in accordance with the invention, the layers can be made stable, in which connection even partial mixing together of layers of different consistencies is prevented. Thus, in the construction in accordance with the present invention, the layers remain in equal proportions, which is important when the individual layers are thin, which is the case with printing papers. The optimal field of application of the invention is a three-layer headbox used in the manufacture of printing papers.
Further, in accordance with the invention, the construction of the headbox is such that the constructional portions corresponding to its individual layers are equal to one another both in respect of the numbers of pipes and in respect of the cross-sectional shapes of the pipes, in which case the flow velocities become equal in each layer.
Thus, since the flow velocities in each layer are substantially equal, no impurity of layers, which might arise from differences in the flows, occurs. Further, in the construction in accordance with the invention, long aprons are used in the slice cone between the layers, which aprons become narrower towards their ends and extend substantially over the entire length of the slice cone. Further, intermediate aprons are used in each layer between the horizontal rows of pipes, which intermediate aprons become narrower towards their ends.
In a multi-layer headbox in accordance with the invention, preferably a so-called single-stock system is employed. In a single-stock system, stock is passed from one common tank and branched into branch ducts, whereby fillers and/or admixtures and/or retention agents required by each layer are added to the basic stock in each branch duct.
In a multi-layer headbox in accordance with the invention, there are at least two layers, preferably three. There may also be more than three layers.
Thus, a basic embodiment of a multi-layer headbox in accordance with the invention comprises a dilution inlet header for providing a flow of diluting liquid, a distribution plate arranged between the inlet headers of the headbox and a tube bank, the distribution plate including stock flow pipes each aligning with a respective tube in the tube bank and flow ducts for enabling the diluting liquid to be passed simultaneously into a plurality of selected ones of the stock flow pipes, conduit means for passing the diluting liquid from the dilution inlet header into the flow ducts in the distribution plate, and regulation means for regulating the flow of the diluting liquid through the conduit means. The regulation means may comprise a plurality of distribution valves arranged across a width of the headbox and the conduit means may comprise ducts each coupling a respective valve to one flow duct in the distribution plate.
In one exemplifying embodiment, the tubes in the tube bank are arranged in vertical columns, each arranged at a specific location along the width of the headbox and including a plurality of tubes, such that the stock flow pipes in the distribution plate are arranged in aligned columns. Each flow duct in the distribution plate is then arranged to enable the diluting liquid to be passed into at least two stock flow pipes in a respective column of stock flow pipes, possibly all of the stock flow pipes in the respective column.
The tubes in each row of tubes should have the same cross-sectional shape and be equal in number such that velocities of the flows of the stocks are substantially equal to one another and differences in speed between the flows of the stocks do not cause impurity of layers.
The multi-layer headbox may also comprise a unique stock supply system including a single stock storage tank, means for forming a separate stock flow for each inlet header from the single stock storage tank and passing the separate stock flows into the respective inlet header, and means for independently adding admixtures and/or chemicals and/or retention agents into the separate stock flows.
In the method for combining a flow of dilution liquid with flows of stock from inlet headers of a multi-layer headbox of a paper or board machine at a location between the inlet headers and a slice cone of the headbox, the stock flow from each inlet header is passed through a respective row of stock flow pipes formed in a common distribution plate and extending across a width of the headbox. The stock flow pipes are arranged in vertical columns, each column being arranged at a specific location along the width of the headbox and including one stock flow pipe from each row of stock flow pipes. Flow ducts are formed in the distribution plate, each flow duct being in flow communication with at least two stock flow pipes in a respective column and having a narrowing duct portion. Dilution liquid is directed through at least one, and preferably all, of the flow ducts formed in the distribution plate and from each flow duct through at least two branch ducts in flow communication with the narrowing duct portion thereof and into a respective one of the stock flow pipes in the respective column. The stock flow from each inlet header may be passed through an inlet plate into the distribution plate.
In one specific embodiment, the stock flows are directed from the distribution plate directly into a tube bank including several rows of tubes, each tube aligning with one stock flow pipe such that the tubes are arranged in discrete rows. The stock flow from each row of tubes is directed into and through an individual equalizing chamber and from the equalizing chambers into the slice cone.
In the method for regulating the basis weight of a web upon its discharge from a multi-layer headbox of a paper or board machine, the multi-layer headbox includes two or more inlet headers, each providing a flow of stock which forms a layer of a web. The stock flow from each inlet header is directed through a respective row of stock flow pipes formed in a distribution plate and extending across a width of the headbox. The stock flow pipes are arranged in vertical columns, each column being arranged at a specific location along the width of the headbox and including one stock flow pipe from each rows of stock flow pipes, i.e., the stock flow pipes are arranged in aligned vertical columns and horizontal rows. Dilution liquid is introduced into the stock flow in at least two stock flow pipes in at least one and preferably all of the columns of stock flow pipes by passing the dilution liquid through a plurality of flow ducts formed in the distribution plate. Each flow duct is in flow communication with at least two stock flow pipes in a respective column and includes a narrowing duct portion having an upstream end and a downstream end such that a cross-sectional area defined at the upstream end is greater than a cross-sectional area defined at the downstream end. Further, the dilution liquid is passed from each flow duct through at least two branch ducts in flow communication with the narrowing duct portion of that flow duct and into a respective one of the stock flow pipes in the respective column. The flow of the dilution liquid through the flow ducts regulated as necessary to enable control of the basis weight profile in the direction of width of the web and the stock flows including the dilution liquid are then discharged from the headbox to form the web.
In one specific embodiment, the stock flows are directed from the distribution plate directly into a tube bank including a plurality of rows of tubes, each tube aligning with one stock flow pipe such that the tubes are arranged in discrete rows, and the stock flow from each row of tubes is passed into and through an individual equalizing chamber and from the equalizing chambers into the slice cone.
In the following, the invention will be described in more detail with reference to the figures in the accompanying drawings. However, the invention is not strictly confined to the details of the illustrated embodiments.